STRATEGY FOR THE FUEL SUPPLY OF AN ENGINE
DIESEL THROUGH SELECTIVE USE OF FOOD MAPS
FUEL TO EXTEND THE HCCI COMBUSTION RANGE
FIELD OF THE INVENTION The invention relates in general terms to internal combustion engines. More specifically, it relates to a control strategy to selectively utilize homogeneous charge compression ignition (HCCI) in a manner that takes advantage of HCCI attributes in different ways during different modes of operation of an engine that It has a variable valve timing. BACKGROUND OF THE INVENTION [0002] Homogeneous charge compression ignition (HCCI) is a known process for fueling a diesel engine in such a way that a substantially homogeneous air-fuel charge is created within an engine cylinder. during an upward compression stroke of a motor cycle. After the injection of a desired amount of fuel for the charge in the cylinder in order to create a substantially homogeneous air-fuel mixture, the increasing compression of the load by the rising piston creates a pressure large enough to cause self-ignition of the load. In other words, the HCCI operation mode of a diesel engine can be defined in the following manner: 1) injection of a desired amount of fuel into a cylinder at an appropriate time during the compression upward stroke such that the fuel injected is mix with the charging air that has entered the cylinder during the preceding intake downward stroke and initial portion of the compression upward stroke so that a substantially homogeneous mixture is formed within the cylinder, and then 2) increasing compression of the mix to the point of auto-ignition near top dead center (TCD) or at the top dead center. Self ignition can occur as the substantially simultaneous spontaneous combustion of authorized fuel at various locations within the mixture. No additional fuel is injected after auto-ignition. One of the attributes of HCCI is that relatively poor or diluted mixtures can burn, keeping combustion temperatures relatively low. By preventing the generation of relatively higher combustion temperatures, HCCI can produce significant reductions in the generation of NOx, an unwanted constituent of engine exhaust gas. Another attribute of HCCI is that self-ignition of a substantially homogeneous air-fuel charge generates a more complete combustion and consequently a relatively lower amount of soot in the engine exhaust. The potential benefit of HCCI in reducing exhaust emissions is therefore relatively significant and therefore HCCI is a subject of active research and development by many scientists and engineers in the engine design and research community. One aspect of HCCI seems to impose a limit on the magnitude with which it can provide drastically reduced exhaust emissions of soot and NOx. At higher engine speeds and higher engine loads, the combustion rate is difficult to control. Accordingly, known engine control strategies can use HCCI only at relatively low speeds and smaller motor loads. At higher speeds and / or more important loads, the engine is powered in such a way that the fuel is burned by conventional diesel combustion
(CD) when injected into the cargo air that has been compressed inside a cylinder at a pressure large enough to cause combustion of the fuel as it is being injected. With the advent of processor-controlled fuel injection systems capable of controlling fuel injection with precision that allows fuel injection at different injection pressures, at different times, and for different durations in the cycle of an engine in the full range of engine operation, a diesel engine may be capable of presenting both CD-type combustion and HCCI-type combustion. As will be explained by the following description, the present invention exploits the capabilities of fuel injection and processing systems to control fuel injections in different ways according to certain aspects of engine operation. The exact form how a particular fuel injection system will be controlled by an associated processing system on any given engine will depend on specific characteristics of the engine, the fuel injection system, and the processing system. Since a diesel engine that drives a motor vehicle operates at different speeds and loads according to various inputs to the vehicle and engine that influence the operation of the engine, the fuel supply requirements change as the speed and load change. An associated processing system processes data indicating parameters such as engine speed and engine load to develop control data to adjust a desired engine fuel feed for particular operating conditions that will ensure proper control of the fuel injection system for Various combinations of motor speed and motor load. The pending United States Patent Application No. 10 / 809,254, filed on March 25, 2004, discloses a diesel engine and fuel injection system controlled by an associated processor that processes certain data to select one of several fuel supply modes to operate the fuel. motor. When a processing result selects a first fuel feed mode (HCCI mode), the engine is fueled during a motor cycle to create a substantially homogeneous air-fuel charge within one or more combustion chambers. This load is compressed to burn by auto ignition, without additional introduction of fuel after auto ignition. When a processing result selects a second fuel feed mode (HCCI-CD 'mode), the engine is fueled during one engine cycle to create a substantially homogeneous air-fuel charge within one or more combustion chambers. This charge is compressed to burn by self-ignition (HCCI), after which more fuel is introduced into the combustion chamber or into the various combustion chambers to provide additional combustion (CD). This engine uses HCCI combustion at relatively lower loads and relatively lower speeds and what is known as HCCI-CD combustion is used at relatively higher loads and relatively higher speeds. SUMMARY OF THE INVENTION The present invention relates to a motor and method of operation for increasing the use of HCCI combustion in a diesel engine towards objectives that include the reduction of the generation of unwanted constituents in engine exhaust, especially hollin and N0, and to improve thermal efficiency. The invention is incorporated into the fuel injection control strategy, a strategy programmed into an associated processing system. According to the principles of the present invention, the use of HCII combustion occurs in a different form than the manner described in US Patent Application Number 10 / 809,254. A first disclosed embodiment of the present invention comprises three different modes of motor operation: 1) an HCCI + RVT mode; 2) an HCCI + VVT mode; and 3) a CD + RVT mode. Each of these modes will be explained in detail below. The HCCI + RVT mode is used in relatively small loads and at relatively low speeds. The HCCI + VVT mode is used at relatively higher loads than the HCCI + RVT mode loads and at relatively higher speeds than the HCCI + RVT mode speeds. The CD + RVT mode is used at loads still relatively larger than the HCCI + VVT mode loads and at speeds still relatively higher than the HCCI + VVT mode speeds. The HCCI + VVT mode allows the benefits of HCCI to be obtained in a portion of the motor operating range between the portion of the range where HCCI + RVT is used exclusively and the portion of the range where CD + RVT is used exclusively. In the HCCI + VVT mode, the closing of the intake valve is delayed in relation to the closing of the intake valve in HCCI + RVT mode. A second embodiment of the present invention offers what is considered HCCI + VVT mode. One way to consider the improvement is by defining the HCCI + VVT mode so that it comprises two modes instead of a single mode. This definition should not necessarily be considered as meaning that the limits, or boundaries of the HCCI + VVT mode remain unchanged. On the contrary, by providing two distinct modes instead of a single mode, it is believed possible to further extend the HCCI combustion useful range, especially in the direction of higher speeds and larger loads. Accordingly, another way of defining the second embodiment of the present invention is through four different modes: 1) HCCI + RVT mode; 2) HCCI + IVC mode; 3) HCCI + IVC + EVC mode; and 4) CD + RVT mode. In the HCCI + RVT mode, the engine is fueled and the valves are operated as in the first mode for the same mode. In the HCCI + IVC mode the engine is fueled and the valves are operated for the HCCI + VVT mode of the first mode where the closing of the intake valve is delayed with respect to the closing of the intake valve in the mode HCCI + RVT to reduce the effective compression ratio. In the CD + RVT mode, the motor is powered and the valves are operated as in the case of the first mode for this same mode. The second modality differs from the first modality because it has the HCCI + IVC + EVC mode. In a range of engine speeds and loads greater than the range during which the bike operates in HCCI + IVC mode, but "less than the range during which the engine operates in CD + RVT mode, the variable timing system The valve operates to delay a closing of the exhaust valves compared to the synchronization of their opening during the HCCI + IVC mode By delaying the closing of the exhaust valves, the percentage of residual hot gases in the cylinders can be reduced, thereby providing a decreased cylinder temperature and pressure, a beneficial result for reducing certain engine emissions such as NOx, for example, a generic aspect of the present invention relates to a method for operating a compression ignition engine, the The method comprises: a) processing certain data to select one of several modes of fuel feeding to operate the engine, b) when a first mode is selected, a limiting in fuel one or more combustion chambers to create a substantially homogeneous air-fuel charge within each combustion chamber of this type during a corresponding engine cycle and compressing each of said charges to self-ignition without introducing additional fuel after self-ignition during this corresponding engine cycle, and c) when a second mode is selected, decrease the effective compression ratio of each combustion chamber of this type compared to the first mode, fuel each of said combustion chambers with a feed in increased fuel compared to the case of the first mode to create a substantially homogeneous air-fuel charge within each combustion chamber during a corresponding engine cycle, and compress each load of this type to auto ignition without introducing additional fuel after auto ignition during this corresponding engine cycle. More specific aspects are presented in the dependent method claims. Another generic aspect relates to a compression ignition engine comprising: a control system for processing data; one or more combustion chambers; and a fuel supply system for injecting fuel into the combustion chamber or into the various combustion chambers; wherein the control system controls the fuel supply system, a) by processing certain data to select one of the various fuel feed modes to operate the engine, b) when a first mode is selected, by feeding in fuel of one or more combustion chambers to create within each of said combustion chambers during a corresponding engine cycle a substantially homogeneous air-fuel charge which is compressed until auto-ignition without introduction of additional fuel after auto-ignition during this cycle of the corresponding engine, and c) when a second mode is selected, by decreasing the effective compression ratio of each of said combustion chambers relative to the effective compression ratio of the first mode, and by feeding the fuel into each one of such combustion chambers with power in increased fuel compared to the fuel feed for the first mode in order to create within each of these combustion chambers during a corresponding engine cycle a substantially homogeneous air-fuel charge which is compressed to the self-ignition point without introduction of additional fuel after auto-ignition during this corresponding engine cycle. More specific aspects are presented in the dependent motor claims. In disclosed embodiments of the present invention, the data processed to select the particular mode comprises motor speed data and motor load data. The injection pressure, duration and synchronization may differ from mode to mode. Data for the various modes are contained in maps in the motor control system. The foregoing, together with additional features and advantages of the invention will be observed from the following disclosure of a currently preferred embodiment of the invention which illustrates the preferred embodiment contemplated at this time to carry out the invention. This specification includes drawings, now briefly described in the following manner. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a representative graph illustrating a fuel feeding strategy in accordance with the principles of a first embodiment of the present invention comprising a combustion mode HCCI + RVT for certain speed-loading conditions, an HCCI + VVT combustion mode for other speed-load conditions and a CD + RVT combustion mode for other speed-load conditions. Figure 2 is a general schematic diagram of portions of an exemplary diesel engine relevant to certain principles of the present invention. Figure 3 is a flow diagram illustrating a strategy according to the first embodiment as used in the engine of Figure 2. Figure 4A illustrates a generic fuel injection in accordance with a generic fuel feed map employed by HCCI combustion + RVT. Figure 4B illustrates a generic fuel injection in accordance with a generic fuel feed map used for combustion HCCI + VVT. Figure 4C illustrates a generic fuel injection in accordance with a generic fuel feed map used for CD + RVT combustion. Figure 5 is an illustrative graph similar to the graph of Figure 1 for the second embodiment of the present invention. Figure 6 is a flow chart illustrating a strategy of compliance with the second embodiment in accordance with what is used in an engine. DESCRIPTION OF THE PREFERRED MODALITY Figure 1 is a graph whose vertical axis represents the motor load and whose horizontal axis represents the motor speed. At the origin of the graph, the motor load is zero and the motor speed is zero. The respective solid lines 50, 52 and 54 separate three zones marked HCCI + RVT, HCCI + VVT, and CD + RVT. RVT means regular valve timing of the engine intake valves, and VVT means variable valve timing of the engine intake valves. The HCCI + RVT zone encompasses an area that includes several combinations of relatively l engine loads and relatively low engine speeds. The HCCI + VVT zone encompasses an area that includes several combinations of relatively larger engine loads and relatively higher engine speeds than in the case of the HCCI + RVT zone. The CD + RVT zone encompasses an area that includes several combinations of still relatively large motor loads and still relatively higher motor speeds than in the case of the HCCI + VVT zone. When a compression ignition engine is operating at a speed and load that is within the HCCI + RVT zone, the fuel is injected into the cylinders of the engine in a manner that creates HCCI combustion. When the engine is operating at a speed and load that falls within the HCCI + VVT zone, the fuel is injected into the cylinders in a way that creates an HCCI combustion. When the engine is operating at a speed and load that falls within the CD + RVT zone, the fuel is injected into the cylinders in a way that creates a CD combustion. Figure 2 schematically shows a portion of an exemplary diesel engine 60 operating in accordance with the strategy of the present invention defined in Figure 1 for driving a motor vehicle. The motor 60 comprises cylinders 62 within which pistons move. Each piston is coupled to a stroke of a crankshaft through a corresponding connecting rod. Admission air is supplied to each cylinder through an intake system when a respective intake valve is open. The engine has a fuel feed system comprising four injectors 64 for the cylinders 62. The engine also has a processor-based engine control unit (ECU) 66 that processes data from various sources to develop various control data for control various aspects of the motor operation. The data processed by ECU 66 can originate from external sources such as several sensors 68, and / or can be generated internally. Examples of processed data may include engine speed, intake manifold pressure, exhaust manifold pressure, fuel injection pressure, fuel supply amount and timing, mass air flow, and accelerator pedal position. The ECU (engine control unit) 66 controls the fuel injection in the cylinders 62 by controlling the operation of the fuel supply system, including controlling the operation of the fuel injectors 64. The built-in processing system in the ECU 66 it can process data at a sufficient speed to calculate, in real time, the synchronization and device drive duration to establish both the timing and the amount of each fuel injection in a cylinder. Said control capability is employed to implement the strategy of the present invention. The engine 60 also has a VVT 70 system controlled by the ECU66. The VVT system can be any of several known types such as for example a "no-cam" type the VVT system can change the timing with which the intake valves for the cylinders operate and consequently change the effective compression ratio of the cylinders of engine as will be explained more fully below. Regardless of how data values are developed for motor speed and motor load, this particular embodiment of the invention employs instantaneous motor speed and instantaneous motor load to select the particular fuel supply mode for the motor, Either 1) the HCCI + RVT mode to create HCCI combustion, 2) the HCCI + VVT mode to create the HCCI combustion, or 3) the CD + RVT mode to create the CD combustion, and then operate the power supply system. fuel to fuel the engine in accordance with the selected fuel feed mode strategy. Alternatively, only the motor load can be used to select the particular mode. Figure 3 shows a flow diagram 71 for the strategy of the present invention in accordance with that executed by the processing system of the ECU 66. The reference number 72 represents the start of the processing executed by the strategy. A step 74 processes engine speed data and engine load data in order to determine which of the three fuel feed modes of Figure 1 should be selected. One way to select the mode is by supplying one or several maps in the processing system to define the three zones and compare data values for instantaneous motor speed and motor load in accordance with the maps. When step 74 selects HCCI + RVT mode, Figure 3 discloses that the fuel will be injected into each cylinder in order to create an HCCI combustion in all cylinders (reference number 76) with regular valve timing RVT (number of reference 77). The operation continues until its completion when the flow chart processing is repeated. When step 74 selects the HCCI + VVT mode, Figure 3 discloses that the fuel will be injected into each cylinder to create an HCCI combustion in all cylinders (reference number 78) with intake valve timing changed by the timing system valve variable 70 (reference number 79). The operation continues until its completion when the flow chart processing is repeated. When step 74 selects the CD + RVT mode, Figure 3 discloses that fuel will be injected into each cylinder to create a CD combustion in all cylinders (reference number 80) with regular valve timing RVT (reference number 81). In the HCCI + RVT mode, the fuel is injected to cause an HCCI combustion using a regular valve timing (RVT). Figure 4A illustrates a generic fuel feed example for the HCCI + RVT mode. The example is characterized by a relatively higher fuel injection pressure zone 82 and relatively shorter fuel injection duration such that the fuel penetrates the cylinders and mixes well with the air prior to auto-ignition and the resulting products of combustion have low NOx and soot emissions. In the HCCI + VVT mode, fuel is injected to cause HCCI combustion. By varying the effective compression ratio of the engine through the use of variable valve timing, HCCI can be achieved at higher loads and higher speeds than in the HCCI + RVT mode. Figure 4B illustrates a generic fuel feed example for the HCCI + VVT mode. The example is characterized by an area 84 wherein the fuel injection pressure is relatively greater than the fuel injection pressure of zone 82 to cause a greater amount of fuel to enter the cylinders, as required due to the increased load and / or higher speed compared to HCCI + RVT mode, and good mixing with air before self-ignition. In the HCCI + VVT mode, the VVT 70 system is operated to reduce the effective compression ratio from the compression ratio of RVT. This results in a decrease in cylinder pressure and a decrease in peak combustion temperature. Accordingly, but dependent on the particular engine load, the fuel injection duration may be relatively longer than in the case of zone 82. Accordingly, the HCCI combustion range is extended to encompass two zones in Figure 1, with low NOx and soot emissions in both zones. In CD + RVT mode, the engine is fueled to cause CD combustion. Figure 4C illustrates a generic example of fuel feeding for the CD + RVT mode. The example shows an area 86 which is characterized by a relatively lower fuel injection pressure, an advanced fuel injection timing, and a longer fuel injection duration than in either zone 82 or 84. In the CD + RVT, the 60 engine can meet high-speed and high-load requirements, with combustion products containing typical NOx and soot emissions. The following relationship demonstrates how VVT can change the effective compression ratio in a diesel engine. By definition, effective displacement + free space volume Effective compression ratio = volume of free space where the volume of free space in a diesel engine is fixed. When the intake valve timing is changed by VVT, the effective displacement of the motor varies, as shown below,
ff-dis = v + a ~ a 8s & ~ l2 - a2 i?
where Veff-d? s is the effective displacement of the motor, B is the diameter of the cylinder, 1 is the length of the connecting rod, a is the radius of the crankshaft, and? it is the timing of the intake valve close, ie the crankshaft angle before the TDC (top dead center, for its acronym in English). It is evident that the fact of delaying the closing timing of the intake valve decreases the effective displacement of the engine, and vice versa. Delaying the intake valve closure timing during the HCCI + VVT mode creates a lower effective compression ratio that allows fuel injection while the pressure in the cylinder is low enough to allow the creation of a homogeneous load that can subsequently ignite by HCCI combustion as the piston approaches TDC (top dead center), while an increased fuel feed provides additional energy input for the increased load and / or higher speed than in the case of the HCCI + RVT mode. The delay in the closing of the intake valve, however, should not be so great that the air-fuel ratio becomes excessively low. The engine control system typically contains multiple fuel feed maps correlated with various combinations of speed and load. In the HCCI + RVT zone of Figure 1, the maps will generally be consistent with zone 82 of Figure 4A. In the HCCI + VVT zone, the maps will generally be consistent with zone 84 of Figure 4B. In the CD + RVT zone, the maps will generally be consistent with zone 86 of Figure 4C. When a cylinder is to be fed in HCCI combustion fuel in the HCCI + RVT mode, the processing system uses a corresponding fuel feed map that provides adequate fuel feed parameters to make a quantity of fuel injected which is consistent with zone 82 for particular engine speed and load. When a cylinder is to be fed in HCCI combustion fuel in the HCCI + VVT mode, the processing system uses a corresponding fuel feed map or corresponding fuel feed maps that offer adequate fuel feed parameters to cause the fuel injected is consistent with zone 84 for the particular speed and load of the motor. When a cylinder is to be fed into CD combustion fuel in the CD + RVT mode, the processing system employs a corresponding fuel feed map or corresponding fuel feed maps that provide adequate fuel feed parameters to cause the fuel to inject is consistent with zone 86 for particular engine speed and load. Figure 5 is a graph whose vertical axis represents the motor load and whose horizontal axis represents the motor speed. At the origin of the graph, the motor load is zero, and the motor speed is zero. The respective solid lines 100, 102, 104 and 106 separate four zones marked HCCI + RVT, HCCI + IVC, HCCI + IVC + EVC, and CD + RVT. RVT means regular valve timing of the engine intake valves, IVC means delayed closing of the engine intake valves, and EVC means delayed closing of the exhaust valves. The HCI + RVT zone encompasses an area that includes several combinations of relatively smaller engine loads and relatively lower engine speeds. The HCCI + IVC zone encompasses an area that includes several combinations of relatively larger engine loads and relatively higher engine speeds than in the case of the HCCI + RVT zone. The HCCI + IVC + EVC zone encompasses an area that includes several combinations of relatively larger engine loads and relatively higher engine speeds compared to the HCCI + IVC zone. The CD + RVT zone encompasses an area that includes several combinations of still relatively large motor loads and still relatively higher motor speeds than in the case of the HCCI + IVC + EVC zone. When a compression ignition engine is operating at a speed and a load that falls within the HCCI + RVT zone, the fuel is injected into the motor cylinders in such a way that an HCCI combustion is created. When the engine is operating at a speed and with a load that falls within any of the zones HCCI + IVC or HCCI + IVC + EVC, the fuel is injected into the cylinders in such a way that an HCCI combustion is formed. When the engine is operating at a speed and load that falls within the CD + RVT zone, fuel is injected into the cylinders to form a CD combustion. In HCCI + RVT mode, the engine is fueled and the valves are operated to create HCCI combustion in appropriate valve timing, which is known as regular valve timing or RVT. In the HCCI + IVC mode, the engine is fueled and the intake valves are operated to create an HCCI combustion with delayed intake valve closing in relation to the intake valve closure in the HCCI + RVT mode with the purpose of reduce the effective compression ratio. The exhaust valve closure does not change materially compared to RVT. In the HCCI + IVC + EVC mode, the engine is fueled and intake valves are operated to create an HCCI combustion with delayed intake valve closure in relation to the intake valve closure in the HCCI ++ RVT mode with the object to reduce the effective proportion and compression and also with delayed exhaust valve closure compared to the HCCI + IVC mode. In a range of engine speeds and loads greater than the engine speeds at which the engine operates in HCCI + IVC mode, but lower than the speeds and loads at which the engine operates in CD + RVT mode, the Variable valve timing operates to delay the closing of the exhaust valves compared to the closing timing of said valves during the HCCI + IVC mode. By delaying the closing of the exhaust valve, the percentage of residual hot gases in the cylinders can be reduced, thus offering a reduced temperature and pressure in the cylinder, a beneficial result to reduce several engine emissions such as NOx. . It is believed that this can provide the expansion of the useful IICCI combustion range. In CD + RVT mode, the engine is fueled and the valves are operated in accordance with what is described above. Figure 6 shows a flow chart 110 for the second embodiment of the strategy of the present invention in accordance with that executed by the processing system of the ECU 66. The reference number 112 represents the start of the processing executed by the strategy. A step 114 processes motor speed data and motor load data to determine which of the four modes of Figure 5 should be selected. One way to select the mode is by providing one or more maps in the processing system to define the four zones shown in Figure 5 and comparing the data values for instantaneous motor speed and motor load in accordance with the maps. When step 114 selects the combustion mode HCCI + RVT, Figure 6 discloses which fuel will be injected into each cylinder to create an HCCI combustion in all cylinders (reference number 116) with regular RVT valve timing (reference number 117 ). The operation continues until its completion when the flow chart processing is repeated. When step 114 selects the combustion mode HCCI + IVC, Figure 6 discloses that the fuel will be injected into each cylinder to create an HCCI combustion in all cylinders (reference number 118) with timing of intake valve delayed by the system variable valve timing 70 (reference number 119). The operation continues until its completion when the processing of the flow chart is repeated. When step 114 selects the combustion mode HCCI + IVC + EVC, Figure 6 discloses that fuel will be injected into each cylinder to create an HCCI combustion on all cylinders (reference number 120) both with the intake valve closure and exhaust valve closure delayed by the variable valve timing system 70 (reference number 121). The operation continues until its completion when the flow chart processing is repeated. When step 114 selects the CD + RVT mode, Figure 6 discloses that fuel will be injected into each cylinder to create a CD combustion on all cylinders (reference number 122) with RVT regular valve timing (reference number 123). The operation continues until its completion when the flow chart processing is repeated. In the HCCI + IVC mode as well as in the HCCI + IVC + EVC mode, the fuel supply is increased compared to the fuel supply of the HCCI + RVT mode. For HCCI + RVT mode, the generic fuel feed map should be similar to Figure 4A; in the case of HCCI + IVC mode, the generic fuel feed map will be similar to Figure 4B; in the case of the HCCI + OVC + EVC mode, the generic fuel feed map will be similar to Figure 4B, but slightly higher and wider than in the case of the HCCI + IVC mode; and for the CD + RVT mode, the generic fuel feed map will be similar to Figure 4C. CD fuel injection during a motor cycle is sometimes described through its particular fuel supply pulses, such as pilot injection pulses, main injection pulses, and post-injection pulses. Any particular fuel injection process always typically comprises at least one main fuel injection pulse, with one or more pilot and / or post injection pulses being optional possibilities. The HCCI fuel feed may comprise one or more individual pulses. The invention has the following advantages: 1) It can concurrently reduce NOx and soot. 2) It has a high thermal efficiency. 3) It can cover the entire range of operation of an engine. 4) Can be used in heavy-duty diesel engines, medium duty and light duty.
) The invention can be implemented in the processor alone, provided that the processor has a sufficient capacity, and this makes the invention quite economical. While a currently preferred embodiment of the invention has been illustrated and described, it will be appreciated that principles of the present invention apply to all embodiments that fall within the scope of the following claims.